1. Field of the Invention
This invention relates to a surface-treating coating composition for plastic articles, and more specifically, to a surface-treating coating composition which finishes the plastic articles to provide scratch-free surfaces and has superior durability in long-term outdoor use.
2. Description of the Prior Art
Transparent plastic articles, for example amorphous shaped articles of thermoplastic resins such as polycarbonate, poly(methyl methacrylate), cellulose butyrate, polyvinyl chloride and polystyrene have heretofore been considered to be useful in many applications as glass substitutes because of their transparency and light weight. They have, however, had only limited applications because they are susceptible to scratch and tend to lose transparency.
In an attempt to remove such a defect, a method has been suggested which involves coating a transparent paint on the surface of a transparent plastic article to form a protective film. However, paints used for this conventional method are either unsatisfactory in scratch resistance, or even when having satisfactory scratch resistance, lack adhesion to a substrate resin or durability in outdoor use which are important properties in actual use. For example, U.S. Pat. No. 3,451,838 suggests a method which comprises coating the surface of an acrylic resin or polycarbonate resin article with a paint composed of a mixture of methyltriethoxysilane and phenyltriethoxysilane, and curing the coating. The plastic article surface-coated by this method, however, has inferior scratch resistance, and therefore, has only limited applications.
U.S. Pat. No. 3,961,977 discloses a method which comprises coating a plastic article with a coating agent composed of a partially hydrolyzed product of an epoxyalkylalkoxy silane and an aminoalkylalkoxy silane and an organic solvent. The plastic article surface-coated by this method has very good scratch resistance, but on the other hand, suffers from low resistance to hydrolysis and to light so that in outdoor use, the coating undergoes cracking and peeling within short periods of time.